In the competitive marketplace which exists for automated surface-mount (SMT) electronics equipment, including systems for fabricating electronics equipment or components, improvements in accuracy and speed are a significant advantage. Such equipment is often used in fabricating, for example, semiconductor chips, printed circuit boards, liquid crystal displays, and thin film devices, and may feature multiple gantry/head assemblies, linear motors, photoimaging systems, etching systems, and/or a number of other technologies. The present invention relates to devices and methods for reducing vibration inherent in such equipment during operation thereby to improve the speed and/or accuracy of such equipment.
For example, modem photolithography tools require extremely high exposure accuracy. This can only be achieved if the levels of elastic displacement at crucial points in the tool do not exceed several nano-meters. Since lithography tools contain numerous moving parts such as the reticle and wafer stages, they are subject to persistent disturbing forces acting on their structure. Moreover, the tool structure is subject to environmental disturbances such as floor vibrations and air turbulence. While the level of these disturbances can be reduced, they cannot be eliminated in their entirety.
There are a number of existing techniques employed to limit the elastic vibration of lithography tools. For example, the stiffness of the structure that supports key elements such as the lens assembly may be increased, tuned mass dampers may be used, the signals applied to the moving stages may be shaped, or the floor vibrations may be isolated using actively controlled air springs. While effective in reducing elastic vibration, these methods often do not meet the stringent requirements of more advanced photolithography tools.
Current efforts to control vibration on SMT placement equipment include placing frictional damping device at the end of the gantry. This xe2x80x9cfriction blockxe2x80x9d serves mainly to stabilize the gantry and head trajectory control system, but it also has been shown to reduce the settling time during certain pick and place operations. However, the effectiveness of the friction block depends on precise tuning of the normal force (or pre-load). The friction block tends to wear out quickly, greatly reducing its effectiveness and contaminating the rest of the machine with particles. Moreover, the friction block works against rigid body movement, resulting in slower operation of the equipment. The vibration control system of the present invention, which comprises an actuator assembly, serves to replace the friction block entirely while improving settling time, or, alternatively, to operate in conjunction with the friction block, providing additional accuracy or speed of operation.
One aspect of the present invention relates to actuator elements useful for active vibration reduction, structural control, dynamic testing, precision positioning, motion sensing and control, and active damping. Electroactive materials, such as piezoelectric, electrostrictive or magnetostrictive materials, are useful in such tasks. In one embodiment of the invention, bare electroactive elements are used. In another embodiment, packaged electroactive elements, as described herein, are used.
Thus, improvements are desirable in the manner in which vibration is controlled in systems for fabricating electronic components, as well as the manner in which an actuator is attached to the equipment to be controlled.
In one embodiment of the invention, a vibration control system is provided comprising an actuator assembly, and a sensor for sensing a parameter of movement or performance. The vibration control system is particularly useful for controlling vibration in systems for fabricating electronics components, which often include one or more gantry assemblies, head assemblies, and/or moving stages or components. Contemplated systems for fabricating electronics components include, but are not limited to, pick and place systems, lithography systems, and those used to fabricate semiconductor chips, printed circuit boards, liquid crystal displays, and thin film devices. However, the devices and methods of the invention would be useful in fabricating systems of any sort, such as machine tool equipment, milling equipment, or systems used in an automated assembly line. Also contemplated are systems for fabricating electronic components wherein the systems comprise a lens system, a wafer stage, and a structure for supporting the lens system and wafer stage where the lens system creates an image on the wafer stage such as would be used in modern photolithography.
In one embodiment, an active vibration control system for use with a photolithography fabricating system includes the following components: a sensor that measures the displacement levels at the key points, or provides information from which such information can be estimated; a digital or analog processor that can compute a control signal based on the sensors input, and an actuator that can induce elastic displacement in the structure.
In a particularly preferred embodiment, an actuator useful in an active vibration control system used in conjunction with photolithography tools is non-reactive and does not require back support (actuators that require back support may excite elastic vibrations in the support structure, which may be re-introduced unto the tool), and has a very low distortion profile (an actuator array designed to control structural vibration at a given frequency or band must not excite any vibration outside that band).
In a particularly preferred embodiment, a vibration control system in accordance with the invention comprises an induced-strain actuator that acts directly on the strain state of the structure, and has virtually no distortion. Such an actuator can excite, and therefore control, only the elastic vibration modes of the controlled structure, leaving all other vibration modes (such as the modes of various equipment housing structures, etc.) uncontrolled. This contributes to the control system simplicity and robustness.
In another preferred embodiment of the invention, the vibration control system further comprises a circuit in electrical communication with the actuator assembly and the sensor. In one embodiment, the sensor relays information about movement, vibration or performance to the circuit, which, in response, signals the actuator assembly to control vibration. The vibration in the systems in which the present invention are useful may be due to external disturbance or due to the inherent disturbances generated by the system itself.
In yet another preferred embodiment of the invention, the vibration control system further comprises an electrical connection to the fabricating system. The electrical connection may provide for the fabricating system to send to, or receive from the vibration control system information such as abling or disabling signals, system status signals, or fault/error status signals. In another embodiment, a circuit according to the invention further comprises a control system comprising at least one controller. Such a control system may permit auto-tuning, gain scheduling, external gain control, or it may be a linear feed forward control, or may serve as another source of feedback control.
In an embodiment of the invention wherein the vibration control system has an auto-tuning control, prior to operation, the control system injects one or more test signals into the system and measures the response. The measured response is used to refine an internal model of the plant, and the control gains are modified accordingly. Control gains are kept constant while the loop is closed.
In an embodiment of the invention wherein the vibration control system has a gain scheduling control, the controllers are designed for the system at several different operating points. In the case of a pick and place machine, these points would be different positions of the pick and place head. The controllers are stored in memory in the digital control system. During operation, sensors feed information to the controller describing the configuration of the machine in real time. As the system moves through each operating point, the control system switches to the optimal control gains for that point. A variant of this is that the control gains used at any point in time are a linear interpolation of the gains from several controllers stored in memory for several nearby operating points.
In an embodiment of the invention wherein the vibration control system has an external gain control, the control system includes an input which connects to the computer system which monitors the overall performance of the machine. The controller implemented at any instant in time has a gain which is proportional to this signal. The monitoring system modifies this gain until optimal performance is achieved. If performance begins to move out of specification due to slow time variation, the monitoring system would repeat the gain optimization sequence.
In an embodiment of the invention wherein the vibration control system has a feed forward control, in addition to the feedback control (controller driven by signals originating from sensors which monitor the structural vibration), an additional signal which is in phase with a harmonic disturbance (such as motor rotation) provided to the controller. The controller feeds forward a filtered version of this signal. The gains which adjust the magnitude and phase of the feed forward control relative to the disturbance signal are adjusted adaptively to minimize the influence of the disturbance on the performance.
In certain embodiments of the invention, the actuator assembly may comprise a strain actuator, an electroactive strain actuator, a piezoceramic strain actuator, an electroactive stack actuator, or at least two actuators. In yet another embodiment of the invention, the actuator assembly is in electrical communication with the sensor.
Also in certain embodiments of the invention, the sensor may comprise a strain sensor, an accelerometer, laser displacement sensor, laser interferometer, or at least two sensors. In another embodiment of the invention, the sensor may comprise at least two sensors measuring at least two different signals. In a preferred embodiment, the sensor directly measures some aspect directly related to performance of the systems in which the present invention is useful.
In a particularly preferred embodiment of the invention, the vibration control system comprises an electronic link or cable providing information about the trajectory of a gantry and head.
An actuator assembly according to the present invention may include one or more strain elements, such as a piezoelectric or electrostrictive plate, shell, fiber or composite; a housing forming a protective body about the element; and electrical contacts mounted in the housing and connecting to the strain element; these parts together forming a flexible card. At least one side of the assembly includes a thin sheet which is attached to a major face of the strain element, and by bonding the outside of the sheet to an object a stiff shear-free coupling is obtained between the object and the strain element in the housing.
In a preferred embodiment, the strain elements are piezoceramic plates, which are quite thin, preferably between slightly under an eighth of a millimeter to several millimeters thick, and which have a relatively large surface area, with one or both of their width and length dimensions being tens or hundreds of times greater than the thickness dimension. A metallized film makes electrode contact, while a bonding agent and insulating material hermetically seal the device against delamination, cracking and environmental exposure. The bonding agent used may be an epoxy, such as B-stage or C-stage epoxy, a thermoplastic, or any other material useful in bonding together the piezoceramic plate, metallized film and insulating material. The specific bonding agent used will depend on the intended application of the device. In a preferred embodiment, the metallized film and insulating material are both provided in a flexible circuit of tough polymer material, which thus provides robust mechanical and electrical coupling to the enclosed elements. Alternatively, the metallized film may be located directly on the piezoceramic plate, and the insulating material may have electrical contacts.
By way of illustration, an example below describes a construction utilizing rectangular PZT plates a quarter millimeter thick, with length and width dimensions each of one to three centimeters, each element thus having an active strain-generating face one to ten square centimeters in area. The PZT plates are mounted on or between sheets of a stiff strong polymer, e.g., one half, one or two mil polyimide, which is copper clad on one or both sides and has a suitable conductive electrode pattern formed in the copper layer for contacting the PZT plates. Various spacers surround the plates, and the entire structure is bonded together with a structural polymer into a waterproof, insulated closed package, having a thickness about the same as the plate thickness, e.g., 0.30 to 0.50 millimeters. So enclosed, the package may bend, extend and flex, and undergo sharp impacts, without fracturing the fragile PZT elements which are contained within. Further, because the conductor pattern is firmly attached to the polyimide sheet, even cracking of the PZT element does not sever the electrodes, or prevent actuation over the full area of the element, or otherwise significantly degrade its performance.
The thin package forms a complete modular unit, in the form of a small xe2x80x9ccardxe2x80x9d, complete with electrodes. The package may then conveniently be attached by bonding one face to a structure so that it couples strain between the enclosed strain element and the structure. This may be done for example, by simply attaching the package with an adhesive to establish a thin, high shear strength, coupling with the PZT plates, while adding minimal mass to the system as a whole. The plates may be actuators, which couple energy into the attached structure, or sensors which respond to strain coupled from the attached structure.
In different embodiments, particular electrode patterns are selectively formed on the sheet to either pole the PZT plates in-plane or cross-plane, and multiple layers of PZT elements may be arranged or stacked in a single card to result in bending or shear, and even specialized torsional actuation.
In accordance with a further aspect of the invention, circuit elements are formed in, or with, the vibration control system to filter, shunt, or process the signal produced by the PZT elements, to sense the mechanical environment, or even to locally perform switching or power amplification for driving the actuation elements. The actuator package may be formed with pre-shaped PZT elements, such as half-cylinders, into modular surface-mount shells suitable for attaching about a pipe, rod or shaft.